Zero dimension semiconductors so-called semiconductor quantum dots causes a remarkable change in characteristics of electrons, in size quantization of a solid object, compared to a quantum well or a quantum wire. In a system in which a carrier is three-dimensionally confined, electron energy levels are discrete in a delta-functional manner, which consequently causes centralization of state density.
Since such characteristics are expressed, a use of semiconductor dots as a laser medium is advantageous in (i) achieving a low lasing threshold, and (ii) improving lasing efficiency by centralizing gain spectra in a narrower region than a bulk material.
These advantages are proved by an epitaxial-grown self-generated quantum dot laser (e.g. See “Semiconductors 28” 832 (1994); N. N. Ledentsov, V. M. Ustinov, A. Y. Egorov, et. al.). In response to the success of lasing by the epitaxial quantum dots, there is an increasing expectation in development of laser using collide or nanocrystal quantum dot of approximately 10 nm or less, which quantum dot expresses quantum size effect.
Particularly, CdSe nanocrystal is one amongst those most frequently studied as a laser medium and as a nanocrystal which exhibit strong quantum confinement. The luminescent spectra of CdSe nanocrystal easily allows varying of colors in the visible band, and allows relatively easy synthesis.
For example, minute ring lasing of Whispering Gallery Mode using nanocrystals put in a minute tube is a known research on lasing from CdSe nanocrystal (e.g. See Phys. Lett 81, 1303 (2002), A. V. Malko, A. A. Mikhailovsky, M. A. Petruska, ppl.).
Further, for example, lasing from CuCl nanocrystals is known as lasing from nanocrystal quantum dots, beside the lasing from CdSe nanocrystals (e.g. see Appl. Phys. Lett. 62, 225 (1993), Y. Masumoto, T. Kawamura, K. Era).
FIG. 17 illustrates a configuration of laser 100 used in lasing described in the “Appl. Phys. Lett. 62”. This laser 100 includes: a sample 101 which is NaCl crystal into which CuCl nano crystals are embedded; and two-dielectric mirrors 102 serving as a resonator.
The effective mean radius of CuCl nanocrystals in the sample 101 is 5.0 nm, and the external dimension of the NaCl crystal is 3.2×5.6×0.58 mm3. The dielectric mirrors 102 have reflection surfaces 102a of 90% in reflectance. These dielectric mirrors 102 are arranged so that the reflection surfaces 102a are in parallel to each other, and that these reflection surfaces 102a interposes therebetween the sample 101 in its thickness direction (in the direction of the thickness of 0.58 mm).
In Appl. Phys. Lett. 62, lasing is confirmed by irradiating the laser 100, which is cooled to 77K, with nitrogen laser light of 337 nm in wavelength at a pulse width of 10 ns. In this lasing, the lasing threshold value Ith in relation to energy density of the nitrogen laser light irradiated is 2.1 MW/cm2.
FIG. 18 shows emission intensity of before and after the lasing threshold. The dotted line shows the emission intensity of a case where the energy density is 0.86 Ith, and the solid line shows the emission intensity in a case where the energy density is 1.08 Ith.
In the lasing disclosed in Appl. Phys. Lett. 62, an exciton absorption band is excited. As such, a large number of excitons are generated, and biexcitons are generated as the result of interaction amongst the excitons. In this process, ionization of the semiconductor quantum dots, so-called Auger process, occurs. Since Auger process is a nonradiation process, it causes a loss of the excitation energy without light emission. Accordingly, the above lasing is disadvantageous in that light emission efficiency is significantly reduced, and that the lasing threshold value increases.
The present invention is made in view of the above described problems, and it is an object of the present invention to provide a laser device and a lasing method which realize lasing utilizing an efficient light emitting phenomena.